MOS type and CCD type image sensors, which incorporate photosensors in matrix, have been in use. Such an image sensor utilizes the electrical charges which are generated in the photosensors exposed to incident light (i.e., incoming light) referred as “light signal”. For example, when a CCD image sensor is exposed, electrical charges are generated and accumulated in the photosensor circuits of the image sensor, and these charges are used as “light signals” to capture an image electronically. In a MOS image sensor, the junction portions of the photodiodes composing photosensor circuits are electrically charged prior to exposure to light, and the amounts of the electrical charges discharged during the exposure are measured as light signals when the photodiodes are recharged.
For the purpose of widening the dynamic range in the detection of light signals, a photosensor circuit which includes a field-effect transistor (FET, e.g., enhancement type n-channel MOS transistor) has been developed. In this photosensor circuit, an FET is connected in series with a photodiode so that the circuit functions to logarithmically compress the output voltage of the photosensor circuit. This function relies on the characteristics of the FET whose resistance changes logarithmically while the electrical current flowing therethrough is relatively small.
FIG. 6 shows such a photosensor circuit. This photosensor circuit 100 comprises a photodiode PD, an enhancement type n-channel MOS transistor Q1 connected in series with the photodiode PD,.an enhancement type n-channel MOS transistor Q2 whose gate is connected to the junction point P (sensor detection terminal) of the photodiode PD and the enhancement type n-channel MOS transistor Q1, and an enhancement type n-channel MOS transistor Q3 connected in series with this enhancement type n-channel MOS transistor Q2. In addition, a capacitor C is connected to the junction point P. The parasitic capacitance of this capacitor is a composite value of stray capacitance created in the photodiode PD, the enhancement type n-channel MOS transistors Q1 and Q2 and the wires which interconnect these components.
A light signal Ls is detected at the photodiode PD and converted to a sensor current Id whose magnitude is proportional to the intensity of the light signal Ls. The enhancement type n-channel MOS transistor Q1, which acts as a load for the photodiode PD, converts the sensor current Id generated by the photodiode PD to a corresponding voltage which is detectable at the sensor detection terminal P as detection voltage Vd.
In this condition, the enhancement type n-channel MOS transistor Q1 in its weakly reversed condition while the sensor current Id is relatively small, provides a MOS transistor resistance that has a logarithmic characteristic, such that the sensor current Id is converted to the detection voltage Vd in accordance with a logarithmic function. Therefore, even if the light signal Ls changes greatly, thereby changing the sensor current Id greatly (in an exponential magnitude), the change of the detection voltage Vd is kept relatively small, and this logarithmic conversion never experiences saturation. As a result, the dynamic range of the output is substantially wide with respect to the range of the input intensity.
Furthermore, the enhancement type n-channel MOS transistor Q2, which defines an output transistor, performs a voltage-current conversion through which the detection voltage Vd is output from the photosensor circuit 100 as a sensor current signal. The enhancement type n-channel MOS transistor Q3 functions as a switch to connect or cut the line of this sensor current signal, which is generated at the enhancement type n-channel MOS transistor Q2, to an external circuit.
Now, a description is made of the operation of this conventional photosensor circuit. As the drain D and the gate G of the enhancement type n-channel MOS transistor Q1 are connected to a common power supply VD (e.g., 5 volts), a charge current Ij flows from the power supply VD through the enhancement type n-channel MOS transistor Q1 to the capacitor C, and the capacitor C is charged while no light signal Ls is detected (the photodiode PD is not activated). Therefore, the detection voltage Vd at the sensor detection terminal P increases to a value near the voltage of the power supply VD, which value represents the initial condition of the photodiode PD, i.e., no light signal detection.
The detection voltage Vd in this initial condition (i.e., the initial value) is set to a value smaller than the voltage of the power supply VD (e.g., 4.5 volts). The reason is that while the capacitor C is being charged, as the detection voltage Vd at the sensor detection terminal P increases near the voltage of the power supply VD, the voltage V (GS) between the gate G and the source S (this voltage equals the voltage V(SD) between the drain D and the source S) of the enhancement type n-channel MOS transistor Q1 decreases. As a result, the impedance between the drain D and the source S increases rapidly, thereby reducing the charge current Ij.
When the photodiode PD of the photosensor circuit 100 in the initial condition detects the light signal Ls, the ,sensor current Id flows through the photodiode PD. As a result, the detection voltage Vd at the sensor detection terminal P decreases logarithmically to a value lower than the initial value in correspondence with the impedance between the drain D and the source S of the enhancement type n-channel MOS transistor Q1 as the intensity of the light signal Ls increases. By measuring the absolute value of this voltage drop in the detection voltage Vd, the light signal Ls is detected. While the sensor current Id through the photodiode PD is proportional to the intensity of the light signal Ls, the detection voltage Vd at the sensor detection terminal P provides the light signal Ls which is generated in a logarithmic conversion because the detection voltage Vd is a value that is a product of the sensor current Id multiplied by the logarithmic impedance of the MOS transistor between the drain D and the source S.
FIG. 7 shows the characteristic relation between the sensor current Id and the detection voltage Vd. As shown in the figure, the value (initial value) of the detection voltage Vd near the initial condition of the photosensor circuit 100 (when the sensor current Id is about 10−12 ampere) is, for example, 4.5 volts. When the sensor current Id increases by an order of magnitude of five digits (when the sensor current Id is about 10−7 ampere), the detection voltage Vd becomes 42 volts. In this way, the photosensor circuit 100 detects the change of the light signal in a range of five-digit magnitude (i.e., 100,000-fold change) as a change of 0.3 volts in the detection voltage Vd. Therefore, this photosensor circuit has a wide dynamic range for the input light signal Ls.
However, this photosensor circuit 100 has a problem of insufficient sensitivity. When the light signal Ls is minute, and the sensor current Id is minuscule (Id =10−12-10−11), the change of the detection voltage Vd is too small to be detectable because the logarithmic conversion of the sensor current Id to the detection voltage Vd is executed for the whole range of the light signal.
As mentioned previously, when the light signal Ls is terminated, and the photodiode PD is turned off, the charge current Ij flows to the capacitor C, increasing the detection voltage Vd at the sensor detection terminal P. However, as the impedance between the drain D and the source S of the enhancement type n-channel MOS transistor Q1 increases rapidly, the detection voltage Vd never increases beyond a predetermined value (4.5 volts). FIG. 8 shows the chronological characteristic of the ascending detection voltage Vd in a broken line L (100). As seen from the characteristic curve, after the turn-off of the photodiode PD, as the detection voltage Vd approaches the predetermined value, the rate of increase of the detection voltage Vd decreases. Thus, a substantial time must elapse before the detection voltage Vd reaches the predetermined value (4.5 volts).
If a plurality of photosensor circuits 100 of this type are arranged in a matrix to compose an image sensor, then the resultant image sensor experiences a problem of after-image. When the image sensor is reset to the initial condition, a relatively long time must elapse for the detection voltage Vd to return to the predetermined value (4.5 volts) as mentioned above. This slow response of the circuits causes the appearance of a residual image on the image sensor.
Also, the photosensor circuit 100 is prone to sensitivity softening. As the enhancement type n-channel MOS transistor Q1 and the capacitor C also work for noise as a peak hold circuit, the circuit mistakenly recognizes a noise which has a large amplitude as a light signal Ls. This condition lowers the SN ratio, and the lowest detectable intensity is impaired by noise. Thus, the sensitivity of the circuit is softened.
To solve these problems, the applicant of the present invention has invented a photosensor circuit which is capable of detecting a substantially weak light signal, which is not prone to generate a residual image, and which has a high SN ratio (see co-pending U.S. patent application Ser. No. 08/925,852, filed on Sep. 9, 1997, now U.S. Pat. No. 5,861,621, and Japanese Patent Application No. H8-239503). FIG. 9 show this photosensor circuit 200, which differs from the above mentioned photosensor circuit 100 on the following points. The drain D of the enhancement type n-channel MOS transistor Q1 is connected to a constant-voltage power supply VD (e.g., 5 volts), and the gate G thereof is connected to a gate-voltage power supply VG, which is capable of supplying two, high and low gate voltages.
In this photosensor circuit 200, the gate G of the enhancement type n-channel MOS transistor Q1 is supplied with a high voltage VH which is substantially higher than the drain voltage VD (i.e., 5 volts) and a low voltage VL which is equal to or lower than the drain voltage VD as the gate voltage VG in the timing shown in FIG. 10. When the high voltage VH is applied as the gate voltage VG, the impedance between the drain D and the source S of the enhancement type n-channel MOS transistor Q1 drops into a low impedance condition. As a result, the capacitor C is charged rapidly, so the detection voltage Vd at the sensor detection terminal P increases to a value (e.g., 4.95 volts) which is almost equal to the drain voltage VD (i.e., 5 volts) as shown by the real line L (200) in FIG. 8. If a plurality of photosensor circuits 200 of this type are arranged in a matrix to compose an image sensor, then the resultant image sensor is not affected by a problem of after-image because the detection voltage Vd returns quickly to the predetermined value (4.95 volts) in response to the resetting of the image sensor.
For the photosensor circuit 200 to detect a light signal, the gate voltage VG is set to the low voltage VL to bring the enhancement type n-channel MOS transistor Q1 into its weakly reversed condition. In this condition, when the photodiode PD is exposed to the light, the electrical charge stored in the capacitor C is discharged. Here, if the intensity of the light hitting the photodiode PD is relatively low, then the sensor current Id does not flow much. Therefore, the enhancement type n-channel MOS transistor Q1 stays in a high impedance condition, so mainly the electrical charge of the capacitor C is used for this small current. As a result, the change of the output voltage is linear. However, in correspondence with the increasing intensity of the light, to which the photodiode PD is exposed, the change of the detection voltage Vd is altered in such a way as shown by an arrow in FIG. 10. If the intensity of the light is relatively high, then the electrical charge of the capacitor C is consumed quickly. As a result, the sensor current Id, which flows through the photodiode PD, is now supplied through the enhancement type n-channel MOS transistor Q1. In this condition, the change of the detection voltage Vd is logarithmic.
This characteristic of the photosensor circuit 200 is shown in FIG. 11. While the intensity of the light is relatively low, the sensor current Id is in a range of 10−12 to 10—11. In this condition, the electrical charge of the capacitor C is discharged, and the detection voltage Vd changes linearly. When the intensity of the light is high, and the sensor current Id is in a range above a value of 10−11, the detection voltage Vd changes logarithmically. In summary, this photosensor circuit 200 provides a linear output characteristic which is equivalent to that of an ordinary MOS type element when the intensity of the light is relatively low (i.e., the sensor current Id is small), and it provides a logarithmic output characteristic which is equivalent to that of a logarithmic type element when the intensity of the light becomes higher (i.e., the sensor current Id becomes relatively large). Thus, this photosensor circuit takes advantage of a cumulative effect when the sensor current is small. Thereby, it realizes high sensitivity and solves the problem of bad SN ratio which affects a logarithmic type element in an insufficient light intensity.
However, this photosensor circuit 200 has a disadvantage of requiring a large number of voltage sources, namely, the high and low voltage sources, which supply the high voltage VH and the low voltage VL to the gate of the enhancement type n-channel MOS transistor Q1 as the gate voltage VG, and the other voltage sources which supply the drain voltage VD for the enhancement type n-channel MOS transistor Q1 and the drain voltage VDD and the gate voltage VC for the enhancement type n-channel MOS transistor Q3. Therefore, the photosensor circuit 200 is complicated in its design.